1. Field of the Invention
This invention relates to mass spectrometers, and in particular to mass spectrometers which incorporate a magnetic sector analyser.
2. Related Art
In a magnetic sector mass spectrometer, a beam of ions is deflected by a magnetic field by an amount dependent on the mass to charge ratio (m/z) of the ions. In such an instrument, ions from a source are first accelerated through an electrical potential V to an energy of EQU zV=mV.sup.2 /2 [1]
where v is the velocity of the ion after acceleration. On passing through the magnetic field, which is disposed perpendicular to the plane in which the ions are travelling, the ions experience a centrifugal force mv.sup.2 /r, where r is the radius of curvature of the path of the ions in the magnetic field. If the magnetic field strength is B, the force exerted by it is Bzv, so that EQU Bzv=mv.sup.2 /r [2]
Combining equations [1] and [2], EQU m/z=(B.sup.2 r.sup.2)/2V [3]
In practice, r is fixed by the use of 2 narrow slits in fixed positions relative to the magnetic field, and V is held constant, so that ions of different m/z ratios are selected by changing the magnetic field B. Thus the effect of the magnetic field can be compared with that of a prism which disperses a beam of white light into its spectral components. A magnetic field can also be arranged to provide a direction focusing effect on a beam of ions, in the same way as does an optical lens with a beam of light. Thus it can be made to form an image of a source of ions at the same time as it separates that beam into its components of different m/z ratios; that is, a series of focused images, each corresponding to ions of different m/z ratios, can be produced. In order to achieve this directional focusing behaviour it is of course necessary to appropriately position the object and image slits and select the shape of the magnetic field, exactly as it is in the case of an optical lens used to produce an optical image. The theory and practice of the methods used are well known. Magnetic sector mass spectrometers which utilize the directional focusing properties of the magnetic field as well as its dispersive properties in order to obtain the sharpest possible image and hence the highest mass resolution are known as single focusing mass spectrometers.
No matter how carefully a single focusing mass spectrometer is designed, however, its resolution is always limited by the spread in the velocity of ions of the same m/z ratio which pass through the object slit into the magnetic field. In practice, the commonly used ion sources produce an energy spread of several electron volts, and the resulting energy variations in the accelerated ion beam (typically 3-10 keV) usually limits the resolution to about 3,000 (10% valley definition). In order to achieve high resolution, it is necessary to use an energy selecting device in conjunction with the magnetic sector analyser. The most common type employed consists of a sector formed of two cylindrical plates spaced a constant distance apart with an electrical potential gradient (E) maintained between them. If the radius of the path of the ion beam between the plates is r.sub.e, then the force experienced by the ions is given by EQU zE=mv.sup.2 /r.sub.e [ 4]
whilst the energy possessed by the ion is given by equation [5], as in the case of the magnetic sector analyser. EQU zV=mv.sup.2 /2 [5]
Combining these equations, it is found that EQU r.sub.e =2V/E [6]
so that an electrostatic sector analyser of this kind disperses an ion beam according to the translational energies of the ions of which it is formed. If r.sub.e is fixed by the use of narrow slits, then the electrostatic sector analyser can be used to select ions of a particular energy from a beam having a significant spread of energies. As in the case of a magnetic sector analyser, an electrostatic analyser can also provide direction focusing of the beam providing that the object and image slits are correctly positioned and the field itself is properly shaped. Use of this focusing behaviour clearly enhances the resolution of the analyser.
High resolution mass spectrometers therefore employ both an electrostatic sector and a magnetic sector analyser in series in order to provide both mass and energy filtration of the ion beam. It is well known that in spectrometers of this type particular combinations of electrostatic and magnetic sectors also result in velocity focusing of an ion beam as well as direction focusing; in other words an ion beam of one m/z ratio entering the first analyser within a certain range of incident angles and having an energy lying within a certain range of values will be accurately focused to the same point on the exit focal plane of the second analyser. Mass spectrometers of this type are known as double focusing mass spectrometers, and are capable of resolutions in excess of 100,000 (10% valley definition). The methods used to design double focusing mass spectrometers are well known in the art. Known spectrometers of this kind fall into two classes. Those having Nier-Johnson geometry, illustrated in FIG. 1, have a geometrical arrangement such that a real, direction focused image is formed by the first analyser, and this image serves as the object of the second analyser. This corresponds to the formation of a real image by a convex optical lens when the object is situated at a distance from the lens greater than its focal length. Similarly, a real image is formed by the second analyser at the detector.
Spectrometers having Mattauch-Herzog geometry, illustrated in FIG. 2, do not form a real intermediate image. Instead, the image of the first sector is arranged to be at infinity, and the object distance of the second analyser is also arranged to be infinity, so that a real image is formed by the second analyser at a distance equal to its focal length. This arrangement in general provides a smaller instrument than the Nier-Johnson geometry for a similar performance and is well adapted to provide an extended focal plane along which a photographic plate or a multichannel detector can be positioned so that the entire spectrum can be recorded simultaneously.
Obviously, the focusing actions described above are imperfect, and suffer from aberrations, as do those of simple optical lenses. Many of these aberrations can be predicted theoretically and can be minimized by further selection of the positions and shapes of the fields and by fixing certain critical dimensions. Additional magnetic and/or electrostatic lenses can also be incorporated to correct certain of the aberrations. Other aberrations in focusing behaviour, particularly those due to the fringing fields at the entrance and exit of the analysers, are difficult to predict but can be minimized by experimental adjustments. Once again, the principles involved in designing spectrometers to minimize the second and higher order aberrations are well known, but it will be appreciated that because many design parameters have to be fixed in order to minimize the predictable aberrations, the number of possible designs for a very high performance double focusing mass spectrometer is limited. For example, Hinternberger and Konig, (in Advances in Mass Spectrometry, vol. I, 1959, P16-35) have given details of a method used for designing spectrometers corrected for image defects to the second order, and have also proposed many of the practical designs which are possible. High performance double focusing spectrometers according to some of these designs are commercially available. In every case they consist of an electrostatic sector analyser and a magnetic sector analyser, and it should be noted that double focusing behaviour can be obtained with the sectors in any order.
A technique of mass spectrometry which is gaining rapidly in popularity is that of tandem mass spectrometry, often abbreviated to MS/MS. It is used to study the fragmentation of ions, which is usually induced by causing them to collide with molecules of an inert gas in a collision cell, producing fragment ions of various mass/charge ratios and kinetic energies. There are several variations of the technique, which is described in detail in "Tandem Mass Spectrometry", edited by F. W. McLafferty, published by Wiley, New York, 1983. A typical tandem mass spectrometry experiment involves the production of a primary ion beam from a sample, filtration of the beam to produce a beam of ions of a particular m/z value, the passage of this beam through a collision gas cell to induce fragmentation of the ions, and the subsequent mass or kinetic energy analysis of the fragment ions. Experiments of this kind yield useful information on the chemical composition of the sample, and can provide a very specific and sensitive method for the determination of trace components in a complex mixture.
It is possible to utilize a conventional two-sector double focusing mass spectrometer for tandem mass spectrometry if a collision cell is inserted between the two sectors and the first sector is used to filter the primary ion beam while the second sector is used to provide a mass or energy spectrum of the fragment ions. However, the method has the disadvantage that spurious peaks frequently appear in the spectrum due to the passage through one or both of the sectors of ions formed by fragmentation processes other than the one under investigation, sometimes occurring in other parts of the spectrometer. The presence of these "artefact" peaks can result in serious errors in the interpretation of the resultant spectrum. It is well known that their occurrence can be minimized by using spectrometers having three or more sectors, and instruments having a wide range of configurations have been constructed. For example, denoting a magnetic sector as B, an electrostatic sector as E, a quadrupole mass analyser as Q, and a high efficiency quadrupole collision cell as Qc, instruments having the following configurations are known:
______________________________________ BEB BEQ BEQcQ EBE EBQ EBQcQ EBEB EQcQ BEEB QQcQ ______________________________________
Details of the various types of instruments can be found in the following references:
(1) McLafferty, F. W., Todd, PJ, McGilvery, D. C., Baldwin, M. A., J. Am. Chem. Soc. 1980, vol. 102, p 3360-63. PA0 (2) Russell, D. H, McBay, E. H., Mueller, T. R., International Laboratory, April 1980, p 50-51.
Of the above, the three sector BEB and EBE combinations comprise a conventional two sector high resolution primary stage and a low resolution single sector mass or energy analyser following the collision cell. If such an instrument is used without the collision cell, so that the primary beam passes into the third sector, the final image is not velocity focused and consequently a lower resolution will be achieved in comparison with the resolution achievable at the velocity focused intermediate image. BEB instruments can also be configured with the collision cell after the first sector, so that a low resolution primary stage and a high resolution double focusing secondary stage are provided. Use of this type of instrument without the collision cell also produces a lower resolution final image than could be achieved with the second stage alone, because the image produced by the first stage is not velocity focused. Of course the resolution can be improved by fitting a narrow slit at the intermediate image position, but this clearly would reduce the transmission efficiency of the instrument and hence its sensitivity.
Four sector EBEB and BEEB combinations have the collision cell situated between the second and third sectors and thus comprise two double focusing spectrometers in series, with the velocity focused image produced by the first stage serving as the object of the second stage. When used without the collision cell, these instruments clearly produce a velocity focused image, but because of aberration in the first stage this is bound to be of lower resolution than the intermediate image unless an intermediate slit is provided, which reduces sensitivity.
Thus it will be seen that there is no advantage to be gained by using any conventional multiple-sector tandem instrument without a collision cell in comparison with a straightforward two sector double focusing spectrometer. Indeed, the resolution, or sensitivity, or both, will be reduced by so doing. This is in marked contrast with instruments constructed according to the present invention in which all sectors co-operate to produce a final velocity focused image.
Another type of spectrometer having EBE geometry has been described by Takeda, T, Shibata, S, and Matsuda, H, in Mass Spectroscopy (Japan), 1980, vol. 28 pt. 3, p 217-226. In this instrument the second electrostatic sector is used only for deflecting low mass ions on to the same detector used for higher mass ions, and is not used to provide any energy dispersive action. Another two stage tandem mass spectrometer in which the first stage is a conventional EB double focusing geometry analyser and the second stage is a cross field EB analyser is described in GB patent publication No. 2123924A. This instrument is similar to the four sector EBEB and BEEB configurations described previously.
Yet another type of multiple sector mass spectrometer has been described by I. Takeshita in review of Scientific Instruments, 1967, vol. 38 (10) pp 361, and in papers referred to therein. Takeshita describes a range of Mattauch-Herzog type spectrometers which comprise two electrostatic sectors preceding a single magnetic sector, which combination can be arranged to produce a velocity and direction focused final image. The object to Takeshita's designs is to overcome a defect of the simple two-sector Mattauch-Herzog design, namely that because no image is formed between the sectors the velocity spread of the ion beam cannot be adjusted independently of the beam divergence. Takeshita's designs require the two electrostatic sectors to be adjacent to one another and for a direction focused image to be formed either between the two sectors, where a slit can be fitted, or inside one of them (in certain special cases where the need for a slit can be obviated). No designs are presented where both those requirements are not met.
A well known difficulty encountered when using a magnetic sector mass spectrometer for organic chemical analysis is the limitation imposed on the speed of scanning the spectrum by the hysteresis of the magnet core. Although there have been many improvements recently made possible by the use of laminated cores and very low resistance coils, the difficulty of relating the actual mass/charge ratio being transmitted to the demanded mass during a fast scan seriously limits the maximum speed attainable. Indeed adequate results can be obtained only through the use of complicated electronic circuitry and by the introduction of reference samples to calibrate the mass scale, sometimes simultaneously with the sample. The selection of suitable reference samples often presents a severe problem. These difficulties could be reduced by using an electromagnet which did not have a ferromagnetic core, but up to now, the strength of the field required to provide an adequate mass range for organic chemical analysis using any of the known double focusing geometries has precluded this. It is an object of the present invention, therefore, to provide a mass spectrometer suitable for organic chemical analysis having double focusing properties which requires a low enough magnetic field to permit the use of a magnet without a ferromagnetic core.